Grave-to-cradle upcycling of Ni from electroplating wastewater to photothermal CO2 catalysis

Treating hazardous waste Ni from the electroplating industry is mandated world-wide, is exceptionally expensive, and carries a very high CO2 footprint. Rather than regarding Ni as a disposable waste, the chemicals and petrochemicals industries could instead consider it a huge resource. In the work described herein, we present a strategy for upcycling waste Ni from electroplating wastewater into a photothermal catalyst for converting CO2 to CO. Specifically, magnetic nanoparticles encapsulated in amine functionalized porous SiO2, is demonstrated to efficiently scavenge Ni from electroplating wastewater for utilization in photothermal CO2 catalysis. The core-shell catalyst architecture produces CO at a rate of 1.9 mol·gNi−1·h−1 (44.1 mmol·gcat−1·h−1), a selectivity close to 100%, and notable long-term stability. This strategy of upcycling metal waste into functional, catalytic materials offers a multi-pronged approach for clean and renewable energy technologies.

mg of solid silica nanospheres. The mixture was stirred under 80 ℃ to evaporate the solvent. The dried powders were further dried under vacuum and then calcined in air (500 ℃, 1h) and H2 (600 ℃, 2h) sequentially.
Acid etching experiments 50 mg of a certain sample (CNC@SiO2@mSiO2 or CNC@mSiO2) was dispersed into 5 mL of HCl solution (1 or 3 M). The suspension was transferred to a shaking bed (30 ℃, 400 rpm) and sampled after 1 h, 3 h, 6 h, 12 h and 24 h, respectively.

The elimination of internal diffusion
The elimination of internal diffusion was verified by the Weisz-Prater criterion (Equation S1), where r represents the reaction rate per volume of catalyst (mol s -1 cm -3 ), R represents the catalyst particle radius (cm), Cs represents the reactant concentration at the particle surface (mol cm -3 ), and Deff represents the effective diffusivity. 1,2 The key to calculate NW-P is to obtain the value of Deff first, which can be predicted from Fuller-Schettler-Giddings method for binary gas phase diffusion.
where DAB is the binary gas phase diffusion coefficient (cm 2 s -1 ), ε is the catalyst porosity, τ is the tortuosity factor, T is the reaction temperature (K), MA and MB is the molecular weight for gas A and B, P is the pressure (bar), ΣVA and ΣVB is the sum of CO2 (unit: mol·g -1 ·h -1 ), and T represents the reaction temperature. All the equations were based on the assumption that the generation of H2 is CO2-free.
The calculations were based on our previous work. The CO rate of ~5 mmol·g -1 ·h -1 was found for SFe-Ni at either a thermocatalytic process (500 °C) or a photothermal catalytic (190 W illumination assisted with a concentrator) process.
Notably, the CO production rate was greatly improved to be ~ 40 mmol gcat -1 h -1 by changing the space velocity from 20000 to 300000 mL gcat -1 h -1 . The slightly lower CO rate of SFe-Ni prepared from real electroplating wastewater rather than SFe-Ni prepared from the synthetic Ni 2+ solution might be ascribed to the slightly higher Fe loading for the latter (17.9 wt%) than the former (15.9 wt%) determined by ICP-OES. As Fig. S7 shows, the Fe component exhibits a significant contribution to the production rate at 500 ℃ in thermocatalytic tests. While at 400 ℃, this contribution is much smaller. That might be the reason for the same CO rate of these two kinds of SFe-Ni samples at 400 ℃. The CO production rate for SFe-Ni-15mg approaches that for SFe-Ni-30mg under the same W/F value (0.00042-0.0015). Therefore, the influence of the external diffusion was eliminated under these testing conditions.
Supplementary Fig. 29 Thermocatalytic performances of SFe-Ni with different particle sizes.
The CO production rate for SFe-Ni (40-60 mesh) approaches that for SFe-Ni (< 80 mesh) under the same condition. Therefore, the thermocatalytic performance would not be dependent on the particle size (< 40 mesh). Combined with the calculations in the supplementary notes, the influence of the internal diffusion was eliminated under these testing conditions.
Supplementary Fig. 30 The relationship between the power of the lamp (P) and the light intensity (I). (a) without a concentrator, (b) with a concentrator.
Supplementary The concentrations of Na, Ca, K, Mn, Mg, Cu and Cr in the wastewater were determined by the ICE3500 atomic absorption spectrophotometer (Thermo Fisher). The concentrations of Al, B, Si, and P in the wastewater were determined by the inductively coupled plasma atomic emission spectrometer (Prodigy, LEEMAN). The concentrations of Ni in the wastewater was determined by an Inductively coupled plasma source mass spectrometer (ICP-MS) (Aurora M90, Jenoptik). Supplementary